Dental Technology - Microwave Acceleration

Dental Technology - MICROWAVE ACCELERATION of DENTAL POLYMER BONDING

Twentieth-century advances in chemistry and the science of materials and their properties produced dental polymers composed of various acrylic resins to replace the organic materials and precious metals used in dentistry, probably as long as recorded human history. Inorganic substances better withstand the long-term stresses on teeth because they are less susceptible to degradation. Human teeth are made of enamel, which is approximately 92% inorganic compound in the form of hydroxyapatite (Ashby & Jones, 1996).

Dental work must remain strong in one of the harshest biological environments: the human mouth. From a strength-of-dental-materials perspective, the mouth is a hostile environment because teeth are regularly subjected to the greatest amount of pressure capable of being produced by the body. Very often, teeth encounter extremely hard substances that result in tremendously increased point-load pressure on a particular tooth, or even worse, only on a small portion of the biting surface area. Teeth are also exposed to the greatest temperature differential encountered in the biological world: they must maintain their structural integrity when subjected to temperatures close to the boiling point of water and resist cracking when exposed to freezing temperatures.

While it is inadvisable to do so, human beings also use their teeth as tools to supplement their hands. The fact that the practice exists irrespective of geography or culture suggests that it is a habit that comes naturally to us. Therefore, by the natural processes of natural selection, the substance of which our teeth are composed have also evolved by virtue of that environmental condition. That is why, generally, the process of creating, refining, and implanting dental prosthetics such as dentures, bridges, and crowns, involves the use of composites of dental polymers. Likewise, in the case of filling cavities in teeth or otherwise repairing damaged teeth, in which case dental polymers are applied instead of amalgam (Ashby & Jones, 1996). Dental polymers provide sufficient strength, resiliency, and dimensional stability to replace natural teeth and approximate their natural functions extremely well. While natural teeth are always preferable to artificial substitutes, dental polymers offer the best artificial alternative because they are biocompatible in humans and chemically inert, and therefore, even less susceptible to organic degradation than natural teeth. Polymers provide the greatest resistance to the biological and mechanical challenges of creating artificial biting surfaces for human teeth within materials capable of being implanted safely for long-term use in the mouth (Ashby & Jones, 1996).

Thermal Application:

The process of applying dental polymers in their various uses in dentistry usually involves the application of thermal radiation as a catalyst to the necessary polymerization of the acrylic resin composites. Polymers applied to the patient's mouth are heated by the manual application of heat or non-thermal radiation applied manually through a tool designed for the purpose of directing energy in that fashion safely. Other methods of achieving polymerization include the use of chemical initiators within the acrylic resins themselves, which require no supplemental energy application.

Dental prosthetics are actually manufactured and refined outside the patient's mouth and then fitted and implanted permanently or fashioned into removable dentures or bridges. Since they are constructed extra-orally, dental prosthetics can be heated to facilitate polymerization without regard to patient comfort or safety issues. Generally the curing phase of the process of manufacturing dental prosthetics involves submerging the viscous non-solid resin matrix monomer material within a plaster-molded imprint of the patient's teeth into a hot water bath for six to eight hours to allow for complete monomer polymerization (Urban, Machado, et al., 2007). The extended reaction time is necessitated by the fact that neither the composites within the polymers themselves nor the clay molds are efficient conductors of thermal radiation. Thermographic photography documents the very gradual increase in temperature from the external surface of the assembly to the entire innermost portions of the polymer molds. However, this gradual rate of temperature increase introduces several factors that compromise the optimal curing conditions for resin-matrix-based polymers.

First, the prolonged curing process allows for the creation of microscopic voids within the material, which decrease overall density and resistance to compression loads of the prosthetics. Second, while the internal heating progresses gradually inside the polymer material during traditional methods of curing through heat application, the slow heat increase also results in uneven thermal conduction that often causes non-uniform polymerization in different areas.

In addition to causing significant shrinkage and dimensional distortion, and thereby compromising the most precise fit possible, the incompletely uniform polymerization accounts for residual monomers that are toxic and capable of irritating oral tissues. Residual monomers may leach into the saliva and act as a chemical irritant to other oral tissues besides those immediately adjacent to the dental prosthetics; it may also diffuse directly into the dentin and the pulp of remaining teeth in the vicinity of the prosthetic (Seghatol & Durand, 1999).

Finally, whereas the traditional prolonged curing method often leaves residual monomers by virtue of incomplete polymerization, it also causes thermal degradation of the external surfaces by virtue of overheating from the prolonged heat exposure required to ensure maximal polymerization of the innermost portions of the prosthetic (Seghatol & (Durand, 1999; Urban, Machado, et al., 2007).

The Advantages of Microwave Curing:

One solution to the problems associated with prolonged heat exposure in the water-bath method has been to use commercial microwaves to accelerate the process, which they do quite well, reducing it from as much as eight hours to a matter of only a few minutes. While the time factor is a substantial convenience in and of itself, the primary functional advantage of curing by microwave instead of water baths is that the microwave energy penetrates the substance much more uniformly and virtually eliminates any differential in heat flow throughout the various layers of the mold, which minimizes thermal lag and thermal gradients, allowing for much more nearly complete and homogenous curing (Seghatol & Durand, 1999)

By definition, the substitution of microwave energy also increases the monomer- to-polymer conversion rate, resulting in stronger intermolecular bonding with less internal density variation and fewer voids that could compromise the strength and integrity of the polymer material over time and after prolonged exposure to dental conditions and stresses. The reduction in the unintended consequences of prolonged, non-uniform heating also results in tighter fittings and better adaptation in patients (Seghatol & Durand, 1999).

While the use of conventional microwave equipment for the purposes of thermal curing of dental prosthetics offers several distinct and significant improvement over the traditional water-bath heating method, the differences in heating uniformity, internal void formation, and shrinkage are merely differences in degree and not complete solutions.

This is mainly a function of the large diameter of the conventional cooking microwave energy beam in commercial microwaves that eventually inspired the incorporation of rotating internal plates, precisely to increase the uniformity of exposure to heating energy.

However, even with the benefit of rotating mechanisms that ensure more nearly uniform heating, the relatively small size of dental prosthetics allows for more error and complicates the goal of achieving completely uniform heating that eliminates the residual byproducts of non-uniform heating, including void formation, incomplete polymerization, and toxic residual monomers after the curing process. Commercial microwave curing is, therefore, only a partial solution to the various problems that can occur during the curing phases (Urban, Machado, et al., 2007). The Evolution of Comprehensive Dental Polymerization Systems:

The different solutions that have been implemented to increase heating uniformity, facilitate complete polymerization, and eliminate residual monomers have mainly arisen in the context of changing the chemical composition of the acrylic matrixes. Specifically, composites have been designed to respond more efficiently to microwave energy and to produce dynamic physical material changes in response to microwave radiation that are more consistent with the characteristics of dimensional retention desired during the curing process. By controlling factors such as microscopic swelling and shrinkage, these biochemical changes are intended to improve the efficiency of curing by microwave radiation (Seghatol & Durand, 1999). Another solution has been to devise mechanical methods of injecting liquid polymer under pressure into pre-heated molds during the microwaving procedure to reduce the overall time required in curing, mainly by eliminating that portion of the delay represented by the heating of the clay mold even under microwave radiation. In that regard, this approach does further reduce the curing time and the additional pressure of the injection process also provides a mechanical force that reduces undesirable dimensional changes in the polymer material during curing (Seghatol & Durand, 1999; Urban, Machado, et al., 2007). These solutions, while improving the efficiency of curing dental prosthetics by microwave, fail to eliminate micro-shrinkage that is inherent in the resin matrix when used in this fashion. In that regard, the extent to which the specific arrangement of molecules in the matrix determines the ultimate degrees of strength, hardness, stiffness, and resistance to abrasion in the final product (Seghatol & Durand, 1999). At the microscopic level, post-polymerized molecules are shorter than residual monomers.

The size and dynamic differential between polymerized molecules and non- polymerized residual monomers results in uneven resistance to compression and weakens the mechanical structure of the denture from within (Seghatol & Durand, 1999). In general, studies of commercial resin matrixes indicate volumetric shrinkage in the curing process as great as seven percent, with most undergoing shrinkage of two or three percent (Seghatol & Durand, 1999).

In dentistry, the comparatively small dimensions of the products composed of polymers and the specific point-load stresses sometimes encountered by dentures magnifies the undesirable effects of even small percentages of incomplete polymerization and volumetric shrinkage. Moreover, dental materials are typically designed to fit with much closer mechanical tolerances than two or three percent if they are expected to perform as designed throughout their intended lifespan (Ashby & Jones, 1996).